Interested in Language
I am confuse about the use of both going to and will.
Suggest you follow emsr2d2's suggestion or Google usage of each.
Forms with BE GOING TO possibly originated in such utterances as:
We are going to meet George at the stadium,
uttered when we were literally going, i.e. on the way, to the meeting. At the moment of speaking there was present evidence of the future meeting. This use has become extended to embrace any action for which there is present evidence – things do not have to be literally moving. Consider now these two utterances:
Look at those black clouds. It's going to snow.
- Pat is going to fly to Phuket next week.
In  the present evidence is clear – the black clouds. In , the present evidence may be the flight ticket that Pat has just shown the speaker, or it may simply be the knowledge in the speaker's mind that s/he has somehow acquired. This explains why, when the grammatical subject of the verb is capable of planning, there may be little practical difference between the use of the 'Progressive’ form and the BE GOING TO form. However, with a grammatical subject incapable of planning, there is a difference:
3. I am meeting my wife at the pub this evening.
3a. I am going to meet my wife at the pub this evening.
- It's going to snow.
7a. It's snowing.
7b. *It's snowing tomorrow.
In  the speaker has made the arrangement. In [3a] the speaker may have made the arrangement (the present evidence), he/she may just have been informed by his wife (the present evidence), or may have recently made a plan (the present evidence). The circumstances surrounding the situations in  and [3a] differ, but the practical result is the same; the speaker has free choice between the two forms. Neither is ‘better, ‘more appropriate’ or ‘more correct’.
In , the present evidence is something like the presence of black clouds, or the speaker's knowledge of the weather forecast. In [7a], it is impossible for an arrangement to be made for future snow, and therefore the Progressive form used here cannot be referring to future arrangement. The context will therefore inform us that snow is actually falling as the utterance is made. The addition of a time-indicator cannot make the impossible possible, and [7b] is therefore not a possible utterance.
We have seen above that if an arrangement of limited duration is what the speaker has in mind, then the example given in the introduction will be realised as:
Emma is flying to London next week.
We can now say that if the speaker has present evidence of next week’s flight, then the example will be realized as
Emma is going to fly to London next week.
The preceding paragraphs help to explain why, in gap-fill exercises, students sometimes come up with a 'wrong' answer, i.e. a different one from that expected by the teacher or compiler of the exercise. In real life, the speaker is trying to say what s/he wants to say; in gap-fill exercises, the student is trying to guess what the compiler would have said in a context that is not made sufficiently clear.
WILL is a modal and, like the other modals, has two core meanings. The two modal meanings are
(a) the 'extrinsic' meaning, referring to the probability of the event/state
(b) the 'intrinsic' meaning, reflectingsuch concepts as: ability, certainty, necessity, obligation, necessity, permission, possibility, volition, etc.
The extrinsic meaning of WILL is exemplified in:
8. Luke left three hours ago, so he will be in London by now.
9. There will be hotels on the moon within the next 50 years.
10. T he afternoon will be bright and sunny, though there may be rain in the north.
In all three examples, the speaker suggests 100% probability, i.e. absolute certainty. (MAY would imply possibility, MUST logical certainty, to take examples of two other modals). Note that while certainty in  and  is about the future, in  it is about the present. It is the absolute certainty, in the minds of speaker/writer and listener/reader, that can give the impression that forms using ‘the WILL future’ are some way of presenting ‘the future as fact’. Some writers therefore call this form ‘the Future Simple’. Weather forecasters, writers of business/scientific reports, deliverers of presentations, etc, frequently use WILL, and students who encounter more English through reading native writers than hearing native speakers informally may assume that it is a 'neutral' or 'formal' future. In fact the particular native writer or speaker is simply opting to stress certainty rather than arrangement, plan or present evidence.
The intrinsic meaning of WILL is exemplified in:I'll carry your bag for you.
- Will you drive me to the airport, please?
- Peter will leave his mobile switched on in meetings. It's so annoying when it rings.
These examples show what we might loosely call volition, the willingness or determination of the subject of the modal to carry out the action. Note that  is not about the future, and in  and  the futurity is incidental. It is context rather than words which gives the meaning.
So, our original example can clearly be realized as:
Emma will fly to London next week.
Without expanded context or co-text, we cannot be sure of what is implied by Emma will fly to London next week. If the background has been that she is scheduled to fly next month, but there is an urgent need for her to be in London soon, the speaker of this utterance is indicating Emma's willingness to fly earlier than intended. In a different context, known to both speaker and listener, the speaker is indicating the certainty of Emma's flight tomorrow, possibly even because of the speaker’s ownvolition. Outside the context of gap-fill exercises this is not a problem.
I'm glad that I just speak it and don't have to understand it.
Thanks all of you
However, sometimes there is a difference. If a manager says to his staff, "I need somebody to come in early tomorrow to open up the office", the natural utterance from a volunteer is, "I'll do it", not "I'm going to do it".
On the other hand, if I explain to my boss why I want a week's leave, I am far more likely to say, "I'm going to have a hip replacement (operation)" than "I will have ...".
In the past, I heard many learners using 'will', sometimes inappropriately, in every reference to the future, because they were taught that this was the future tense in English. More recently, I have found some learners using .BE going to' on every occasion, also sometimes inappropriately, because they have apparently been told that this is the more modern, natural way of expressing the future.
It's not the end of the world if learners use an inappropriate way of expressing futurity, but I believe that they should be encouraged to be as accurate and natural as possible.